The activity of serum ribonuclease (RNase) was measured in 29 patients with various types of leukemia and differing degrees of renal insufficiency. RNase levels are not a biomarker for the extent of leukemia. However, a strong inverse correlation was observed between the degree of renal impairment (expressed as creatinine clearance) and serum RNase levels. Time course studies of eighteen leukemia patients also indicated that RNase levels reflect the status of renal function and not the extent of leukemia. Levels of this enzyme, therefore, are not of use in determining the response of neoplasia to treatment. Enzyme concentrations, however, are useful in determining the degree of renal insufficiency and should prove particularly useful in cases where 24 hour urine collections are difficult, or where a creatinine deficit is suspected. The enzyme responsible for enhanced RNase activity as glomerular filtration rate declines is shown to be antigenically similar, if not idential, to the enzyme found in normal human plasma. At present, determinations of serum RNase levels in patients with Lupus Erythematosis are in progress, with the intent of proving that the elevations in enzyme concentrations reflect renal status and are not a biomarker for the disease itself. BIBLIOGRAPHIC REFERENCES: Karpetsky, T.P., Humphrey, R.L. and Levy, C.C.: Influence of Renal Insufficiency on Levels of Serum Ribonuclease in Patients with Multiple Myeloma. J. Natl. Cancer Institute 58: 875-880, 1977. Humphrey, R.L., Karpetsky, T.P., Neuwelt, E.A. and Levy, C.C.: Levels of Serum Ribonuclease as an Indicator of Renal Insufficiency in Patients with Leukemia. Cancer Research 37: 2015-2022, 1977.